Such a Magnus-effect rotor is known from U.S. Pat. No. 4,602,584. It has long been known that a circular cylinder rotating about its longitudinal axis is capable of producing a lift force when placed in an air stream flowing perpendicular to the longitudinal axis of the cylinder, quite similar to the lift force produced by a wing when placed in a laminar air flow. This lift force is named after its discoverer, Heinrich Gustav Magnus, the German scientist who first investigated this phenomenon in 1853.
The Magnus-effect was first applied for propelling vessels in 1924 by Anton Flettner. Flettner used elongated cylinder structures, standing upright from the deck of the vessel, for propelling the vessel using the lift force mentioned (these structures were also called: “Flettner-rotors”). The advantage with respect to conventional sails was that the vessel was able to sail at sharper angles with respect to mildly opposing, thus relatively unfavourable, wind directions. Additionally, the Flettner-rotor was able to supplement the propulsion of fuel-powered vessel, thereby decreasing the fuel consumption of such a vessel.
However, in case of strong opposing winds essentially parallel to the desired sailing direction, or in wind conditions with severe gusts, the Flettner-rotor fails to provide any additional propulsion. In these conditions the rotor proves to be a great source of drag to due to the wind hitting the relatively large frontal surface of the rotor. Furthermore, in severe weather conditions with strong winds and high waves, the rotor proves to provide additional instability to the vessel due to the raised centre of gravity.
U.S. Pat. No. 4,602,584 provides a Magnus-effect rotor for use onboard a vessel, which offers the possibility of collapsing the rotor towards the deck, by pivoting it to a position essentially parallel to the longitudinal axis of the vessel, for minimizing the drag on the vessel in such unfavourable wind conditions. More specifically, U.S. Pat. No. 4,602,584 describes an elliptical cylinder, having a major and a minor axis, and a longitudinal axis, upstanding from the deck of the vessel and formed from a rigid outer surface which is rotatable about a central longitudinal axis. The elliptical cylinder can be “parked” in a generally upright position where the cylinder can act as a conventional sail, aligning the minor axis with the wind, or in a position where the elliptical cylinder can be feathered in the wind, aligning the major axis with the wind, thereby reducing drag. Furthermore, during unfavourable wind conditions the elliptical cylinder, or rotor, can be parked in a position where it is essentially parallel to the longitudinal axis of the vessel, thereby not interfering in any way with the maneuvering or propulsion of the vessel.
A drawback of folding such a rotor towards the deck is that as the end plate has a relatively large diameter, thereby taking up a lot of space, deck operations can be hampered. Furthermore, the folding operation itself is interfered with by the large end plate diameter. Also, when standing upright from the deck in unfavourable wind conditions or heavy winds in general, such a rotor end plate has a large wind resistance in view of its large surface area and is therefore susceptible to relatively high forces.
Another patent publication that describes a stowable rotor is GB 2.187.154. This publication describes a rotor for ship propulsion, which is constructed as a series of cylindrical sections increasing stepwise in diameter from section to section up the height of the rotor so that the rotor can be retracted telescopically into a well in the ship's deck. A central non-rotating support post inside the rotor is fitted with a top bearing that carries the rotor, the post also being telescopic. The topmost rotor section has a boundary layer fence projecting radially around its top end, and the step increase in diameter between each of the sections below and the section above it provides a respective boundary layer fence for the rotor section below in each case. No motor drive can be found in the publication for actively rotating the rotor around its longitudinal axis as required for generating a propulsion force on the vessel. The known vessel does not provide for effectively stowing the rotor on the vessel in a compact inoperational state in which it has minimal wind interference.
Yet another patent publication that describes a stowable rotor is U.S. Pat. No. 2,596,726. This patent publication describes a wind motor for driving a ship's propeller. More specifically, it describes a motor standing on a bed constructed within the hull of a ship. Stay-lines are employed for keeping the rotor erect relative to the ship. The motor furthermore includes a base plate fastened to the ship's bed by bolts and a mast socketed at its lower end in a boss formed on said base plate, the mast being fitted at its upper end with a spider bracket. The motor further includes a rotor which turns freely around the mast. The rotor comprises a tubular rotor shaft encircling the mast and upper and lower disc-like end plates axially keyed to the rotor shaft. No drive means for actively rotating the rotor around its longitudinal axis are disclosed that are required for generating a Magnus-effect. The rotor is actually being used to provide power to the propeller of the ship. The rotor itself fails to provide a driving force on the vessel. No possibility of stowing the rotor in a compact inoperational position is disclosed.
In view of the foregoing, it is an object of the invention to provide for a Magnus-effect rotor, where the space taken up by the end plate during and after folding or retraction of the rotor is minimized.